The distribution of fatty acid (FA) in cells of blood vessels and brain is readily visible by both light and electron microscopy when 0.1 to 0.3 ml of linolenic acid are infused intravascularly in very high concentrations for ready detection, over a period of 10 minutes. At the end of this time, the vessels and brain are fixed either by aldehydes and osmium or, by osmium solutions only. Not only are FA droplets randomly distributed throughout endothelial and smooth muscle cells, but in some vessel segments, most of the droplets are within pinocytotic pits and vesicles. Normally, FA circulates within the blood in combination with serum albumin and the droplets within pits that take up protein may represent the FA-albumin complex. Within the cerebral parenchyma, the synaptic vesicles of a few axon terminals contain FA droplets and the postsynaptic web in the dendrites of some areas, are heavily stained by the linolenate.